Faulty filter detection technique

Abstract

To locate a defective one of filter/voltage arresters (20 1 , 20 2 , 20 3 , 20 4 , . . . 20 n-1 ), each associated with a corresponding one of splice cases (18 1 , 18 2 , 18 3 , 18 4 , . . . 18 n-n ) on a buried fiber-optic cable (10), an operator employs a locating apparatus (24) to detect the presence of an AM signal within a prescribed frequency range that is radiated when the filter/voltage arrester becomes defective. By determining the location along the cable where the AM signal has its greatest strength, the operator can accurately establish the location of the defective filter/voltage arrester, thereby avoiding unnecessary excavation of each splice case.

What is claimed is: 
     
       1. A method for locating a defective filter/voltage arrester associated with a splice case on a buried fiber-optic cable that carries a locate signal on its sheath, comprising the steps of: sensing, above ground, the presence of an Amplitude-Modulated signal radiated in the frequency range of 400-900 kHz from a defective filter/voltage arrester;   determining, the location, at which the Amplitude Modulated signal has its greatest strength by eliminating extraneous signals below the 400 kHz frequency range; and   excavating at the location at which the Amplitude Modulated signal has its greatest strength to locate the defective filter/voltage arrester.   
     
     
       2. The method according to claim 1 wherein the locate signal on the sheath of the fiber-optic lies in a frequency range of approximately 9 Hz to 33 kHz and wherein the Amplitude Modulated signal lies in the frequency range of 400-900 kHz. 
     
     
       3. The method according to claim 1 wherein the sensing step comprises the step of displacing a directional coil, adapted to receive RF signals within a narrow foot print, along a path of the cable. 
     
     
       4. The method according to claim 3 wherein the path of the cable is determined from a map. 
     
     
       5. The method according to claim 3 wherein the path of the cable is determined from prior radio detection data. 
     
     
       6. Apparatus for locating a defective filter/voltage arrester associated with a splice case on a buried fiber-optic cable that carries a locate signal on its sheath, comprising: a directional antenna for detecting, within narrow footprint, an Amplitude Modulated (AM) signal of a prescribed frequency range that is radiated by a defective filter/voltage arrester when the fiber-optic cable carries a locate signal on its sheath;   an AM receiver coupled to the antenna for receiving said AM signal when said AM signal is above a prescribed strength; and   an audio transducer coupled to said AM receiver for providing an audible indication of the strength of the AM signal received by said receiver.   
     
     
       7. The apparatus according to claim 6 wherein the AM receiver is tuned to receive AM signals in the frequency range of 400-900 kHz radiated by a defective filter/voltage arrester when the locate signal on the fiber-optic cable is within the frequency range of 9 Hz to 33 kHz.